In terms of the technology utilized, printers fall into two categories: LED printers and laser printers. The LED printer uses a light-emitting diode array as a light source in the print head. The printer head with the LED array is usually cheaper than the printer head with the laser due to its simplicity and a longer lifespan. In addition, the mechanical reliability, the compact design and the fewer moving parts of the LED printer makes the LED printers more popular than the laser printers.
However, in the LED printer, the lens assembly used in the print head has very short focal length. The distance from light emitters to the focus point on the photoconductive drum is only about 0.5 mm to about 1 mm. Developers usually design a protection mechanism in the LED printer for preventing the damage of the photoconductive drum on the toner cartridge caused by the collision between the print head and the photoconductive drum during replacing the toner cartridge. The protection mechanism is, for example, a multi-bar linkage or a security lock mechanism.